As automobile design has become more sophisticated, the level of noise inside the vehicle (so-called interior noise) has been dramatically reduced, to the point that in at least one case it is said is the loudest sound is the ticking of the clock. Sources of noise which were previously ignored have now become relatively more significant, and it has become desirable to reduce the interior noise from such sources. The noise of the fuel pump is relatively low but the fact that it operates continuously whether or not the vehicle is moving, makes its noise level more noticeable when other sources are inactive. Indeed, servicing of fuel pumps to reduce their contribution to interior noise is becoming a significant component of warranty service on new cars.
Fuel pumps are typically housed in a sealed, removable "sender unit" which is suspended inside the fuel tank itself. In addition to the pump, the sender unit also includes a pressure or feed line and a return line for returning fuel not consumed at the engine. Some sender units also include a "pulsator" between the pump and the pressure line, the purpose of which is to minimize the transmission of pump pressure pulsations to the pressure line.
When a fuel pump is operating it produces forces at its fundamental operating speed and at the harmonics of that speed. Those forces are converted into vibrational energy which is the source of noise inside the vehicle. The fuel pump vibration, which varies in both amplitude and frequency depending on the specific configuration of the pump, lines, tank, and vehicle body, is primarily transmitted from the pump to the sender unit through the pressure line and/or the return line, through a mounting cap to the tank and the vehicle body where it contributes to the interior noise.
The usual approach to controlling pump noise has been to reduce the level of vibration caused by the pump. This has been a particularly difficult problem to solve in practice. Elaborate attempts have been made to reduce pump vibration, including isolating the pump, damping the resonant vibration, and fine tuning the geometry of the lines at different locations. However, such attempts have been only partially successful because the dynamics of the system (pump, lines, tank, body, and interior cavity) are very complex and any change (even small) in the assembly, its age and operating conditions can tend eventually to minimize, or even negate any improvement. For example, previous attempts to isolate the pump vibration forces have resulted in good reduction in the vertical direction, but not in other directions. Vibrational energy is transmitted in all directions from the pump, that is, longitudinally of the pump axis as well as rotationally and laterally, and it is not practical to isolate vibration in directions other than vertical. For this and other reasons, prior attempts to reduce pump noise have proven insufficient, and there has been a need for a better way to reduce the noise component generated by the fuel pump in a vehicle.